"Resources are one piece of the puzzle; it would be ideal if going into the night we had an extra bed to admit the unexpected patient without having to discharge another."

Dr Gantner said the responsibility for having a bed available is shared by clinicians managing the ICU, by hospital administrators co-ordinating ward beds, and by policy-makers directing the resources.

Previous studies have suggested a link between timing of ICU discharge and hospital outcomes.

Dr Gantner said there are several possible explanations for why this might be.

"One is that patients discharged after-hours may be more sick than patients discharged in-hours. Our study showed that this may account for some of this increased risk, but not all of it.

"The second is that the degree of nursing and medical monitoring available in ICU cannot be provided in normal hospital wards overnight, so if a patient becomes unstable after ICU discharge this may not be picked up until it's too late.

Dr Gantner, who is also an Intensive Care Fellow at Melbourne's The Alfred Hospital, said a third possible explanation is that patients who are expected to die despite ICU-level care may be discharged after-hours to receive palliative care on the ward.

Further research is needed to explore these possibilities, he said.

The study, published in Intensive Care Medicine, was done in collaboration with the Australian and New Zealand Intensive Care Society and involved data from 700,000 patient admissions, between 2005 and 2012.